Measles was the source of a substantial burden in the pre-vaccination era. It is now largely controlled with vaccination in the high-income countries but remains a major cause of child mortality in sub-Saharan countries. The current vaccination strategies provide sub-optimal results in regions where environmental drivers of measles dynamics differ substantially from those in high-income countries. A better understanding of measles dynamics in these settings is necessary to adapt immunization strategies efficiently and reduce ongoing transmission.
We analyzed reported measles cases at the district level in the West African nation Niger from 1995 to 2004 to identify underlying transmission mechanisms. We looked for associations between reported measles cases and environmental drivers associated with human activities, particularly rainfall and agriculture.
We used wavelet decomposition to screen for periodicities. We also used hierarchical clustering and phase shift analyses to look for spatiotemporal patterns in measles transmission. Additionally, we interpolated weekly cumulative rainfall and weekly average temperature with spatiotemporal kriging. We then used quasi-Poisson regression with distributed lags to quantify the association between measles incidence and the rainy season.
Results/Conclusions
We identified two main periodicities: the annual periodicity, strong and consistent, and a second one for 2-3.5 years, weaker and inconsistent. The annual periodicity led to spatial clustering that was consistent with the latitudinal gradient of population density, which reflects the geography of Niger. This pattern was mostly consistent over time. The second periodicity displayed spatially fragmented clustering that was inconsistent over time. The strong regularity of the annual periodicity could reflect the seasonal variation of population density associated with agricultural activity. It illustrates a strong seasonal forcing but also highlights a potential window of opportunity for interventions. The second periodicity likely reflects multiple mechanisms that could cause chaotic disease dynamics, such as reintroductions and vaccine coverage heterogeneity. The rainy season was associated with a lower risk of measles case reporting. It further supports the role of seasonal agricultural labor migration with fluctuating population density.
Tailoring targeted interventions to improve vaccine coverage in migrating populations could efficiently break the strong seasonality of measles outbreaks in this setting. Addressing the more chaotic component of measles dynamic would require regional improvements of access to care to increase vaccine coverage at a metapopulation scale.